Polysilicon of lowered resistance plays an increasingly important role in silicon integrated circuit technology, as, e.g., resistors and conductors. Polysilicon resistors are finding increasing application in RAMs and PROMs, as well as in logic circuits. The same resistors are employed for current limiting in CMOS circuits, as well as in precision DAC-ADC for resistor networks.
Typically, elements of polysilicon of lowered resistance, such as polysilicon resistors, or other active semiconductor elements such as diodes and transistors, are formed through a polysilicon deposition/annealing process, which includes a first oxide layer deposition process to isolate the starting material on which the polysilicon resistor is to be formed. Over the isolating oxide layer a polysilicon layer is deposited, for example, by low pressure chemical vapor deposition (LPCVD) techniques. Impurities are introduced into the polysilicon so deposited through doping by diffusion or implantation, and subsequently annealed. Thereafter, standard integrated circuit contact processing is effected to complete the polysilicon resistor device.
However, resistors formed by these prior art methods are unsatisfactory due to a number of defects. Particularly, the tolerance in such resistors is difficult to control. This is due, in part, to variation in grain size in the polysilicon, and in part to undesired diffusion of the doped or implanted impurities in the polysilicon. These prior art resistors are also characterized in that they exhibit a large, negative temperature coefficient of resistance (TCR) and voltage coefficient of resistance (VCR).
In an attempt to overcome and avoid these defects, processes employing laser beams or similar high energy beams to anneal the deposited polysilicon have been followed. These processes are generally identical to the prior art processes described above up to the deposition of polysilicon over the oxide isolation layer deposited. The polysilicon is then annealed through laser irradiation, or irradiation by a similar high energy beam. Thereafter, the irradiated polysilicon is implanted with dopant, the implant annealed and standard contact processing followed to produce the finished device. Laser irradiation through these processes has allowed excellent control over resistor parameters and improved the TCR and VCR for laser annealed polysilicon resistors.
In order to incorporate the laser annealed resistor process in the silicon integrated chip manufacturing process, for instance, a chip comprising a dielectrically isolated high frequency bipolar circuit, formation of the resistor after the formation of the dielectrically isolated islands making up the circuit is necessary.
Unfortunately, when using the laser annealing process that is described above, a polysilicon resistor cannot be satisfactorily formed over the interisland areas, which is highly desirable to increase circuit density. A polysilicon LPCVD layer is formed over the isolating oxide layer overlying the isolated islands. The polysilicon is then irradiated. However, owing to different laser beam absorption characteristics in the layers underlying the polysilicon film, the polysilicon recrystallizes only over the dielectrically isolated islands, but not over the polycrystalline interisland areas, where recrystallization is also desired. This prevents fabrication of the resistors over the interisland areas.
These absorption characteristics, and the problem encountered in forming polysilicon of lowered resistance, may be better understood with reference to FIGS. 1-3. FIG. 1 is a cross-sectional representation of a semiconductor device bearing a polysilicon thin film. FIG. 2 is a representation of the reflective characteristics over the dielectrically isolated islands of FIG. 1, while FIG. 3 is of the interisland area.
As can be seen by comparing FIGS. 2 and 3, the reflection R of irradiating, high energy beams over the dielectrically isolated islands is much greater than that over the polycrystalline support of the interisland area thereby defining regions of higher and lower high energy reflection characteristics, respectively. Accordingly there is more energy for recrystallization of the deposited polysilicon over the dielectrically isolated island than over the interisland areas. This reflective differential is so great that to recrystallize the polysilicon over the interisland area would require irradiation of such magnitude as to destroy the dielectrically isolated islands exposed thereto. Accordingly, formation of polysilicon of lowered resistance over the interisland areas has been difficult, if not impossible, to achieve in an integrated process, thus sacrificing circuit density.
Although a polycrystalline support bearing dielectrically isolated islands has been used as an example, it will be apparent that the same obstacle will be encountered whenever it is desired to form a layer of laser-annealed polysilicon over a substrate having regions of lower and higher high energy reflection characteristics.
Accordingly, one object of this invention is to incorporate a method for forming polysilicon of lowered resistance in the silicon integrated chip manufacturing process.
Another object of this invention is to provide a process whereby laser annealed polysilicon may be fabricated in the interisland area between dielectrically isolated islands.
Yet another object of this invention is a method which allows the formation of integrated circuit devices of improved circuit density.
A further object of this invention is to provide a method whereby resistors of improved tolerance, TCR and VCR may be fabricated directly on a silicon integrated chip.
Still another object of this invention is to provide a method whereby a dielectrically isolated high frequency bipolar circuit may be formed.
Yet a further object of this invention is to provide a method for formation of laser annealed polysilicon which may be used in the fabrication of active semiconductor devices.